**Core Concept**
Glycogen metabolism involves the breakdown and synthesis of glycogen, a complex carbohydrate stored in the liver and muscles. This process is regulated by various enzymes, including those that catalyze the phosphorylation and dephosphorylation of glycogen synthase and phosphorylase.

**Why the Correct Answer is Right**
Glycogen synthase C is not a recognized enzyme in glycogen metabolism. Glycogen synthase is a key enzyme involved in glycogen synthesis, and it exists in two main forms: glycogen synthase I (active form) and glycogen synthase D (inactive form). Glycogen phosphorylase B is the inactive form of glycogen phosphorylase, which is involved in glycogen breakdown. Glycogen synthase I is responsible for adding glucose molecules to the glycogen chain, whereas glycogen phosphorylase breaks down glycogen into glucose-1-phosphate.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycogen phosphorylase B is indeed an enzyme involved in glycogen metabolism, specifically in the breakdown of glycogen.
**Option B:** Glycogen synthase I is an active form of glycogen synthase, which is crucial for glycogen synthesis.
**Option D:** Glycogen synthase D is an inactive form of glycogen synthase, which is involved in regulating glycogen synthesis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that glycogen synthase is regulated by phosphorylation and dephosphorylation, with the active form (I) promoting glycogen synthesis and the inactive form (D) inhibiting it.

**Correct Answer:**
✓ Correct Answer: C. Glycogen synthase C